Control device for bulldozer blade and its control method

ABSTRACT

A control device and its method by which interference between a blade (10) of a bulldozer and a vehicle body can be prevented even when a pitch back angle of the blade (10) is made large and a tilt speed is increased to greatly change a tilt angle. Therefore, the device comprises left and right detecting means (20a, 20b, 23a, 23b) for detecting an amount of tilt of the blade (10), left and right tilt limiting valves (37, 38) connected to operating units (35a, 35b) of first and second directional control valves (35, 36), and a controller (50) outputting a command to the left and right tilt limiting valves (37, 38) so as to stop a tilting action of the blade (10) when a difference between amounts of tilt detected by the left and right detecting means (20a, 20b, 23a, 23b) reaches a predetermined limit value.

FIELD OF THE INVENTION

The present invention relates to a control device, for automaticallylimiting an operation angle into a tilt position for a blade of abulldozer and for preventing interference between the blade and thevehicle body, and its control method.

BACKGROUND ART

Conventionally, a blade 10 of a bulldozer is removably installed so asto pivot in the forward and backward directions at respective ends ofthe left and right frames 15A and 15B as shown in FIGS. 6 and 7A to 7C.One end of each of the left and right hydraulic lift cylinders 21A and21B is removably and pivotably installed at the back of the blade 10 andthe other end of each of these cylinders is removably and pivotablyinstalled in a vehicle body 20x. The blade 10 is moved upwardly anddownwardly by a contraction/expansion driving of the left and right liftcylinders 21A and 21B.

One end of each of the left and right hydraulic pitch cylinders 20A and20B is removably and pivotably installed at the back of the blade 10 andthe other end of each of these cylinders is removably and pivotablyinstalled in a respective one of the frames 15A and 15B. If the left andright pitch cylinders 20A and 20B are simultaneously extended fordriving, the blade 10 is put into a pitch dump position (leaningforwardly) for dumping earth and sand. If the left and right pitchcylinders 20A and 20B are simultaneously retracted for driving, theblade 10 is put into a pitch back position (leaning backwardly) forcarrying earth and sand. If the blade 10 has a nose angle α of 55degrees, the blade 10 is in an excavation position for excavating earthand sand.

Furthermore, if the right pitch cylinder 20B is stopped without beingsupplied with any pressurized oil while only the left pitch cylinder 20Ais supplied with pressurized oil for extension, the blade 10 makes arightward tilting action (the right end of the blade 10 tiltsdownwardly). On the contrary, if the left pitch cylinder 20A isretracted, the blade 10 makes a leftward tilting action (the left end ofthe blade 10 tilts downwardly).

A request has been made for increasing the amount of carried earth andsand, by enlarging the pitch back angle of the blade 10, since abulldozer carries the earth and sand for a long distance in a work oftopsoiling, such as a strip mining. Therefore, the blade 10 is made withthe maximum pitch back angle of approximately 45 degrees, at which theblade 10 is close to the vehicle body 20x.

However, if the blade 10 is operated with a tilting action from a stateof the maximum pitch back angle, the blade 10 further approaches thevehicle body 20x as shown in FIG. 8, and then it interferes with aradiator guard or the like in front of the vehicle body. Thisinterference easily occurs since the blade 10 exceeds a tilt angle limitvalue while it is operated at a higher tilt speed. On the other hand, ifthe blade 10 is operated at a lower tilt speed, it stops before anoperator achieves the desired tilt angle, and therefore the operatorcannot achieve the desired tilt angle.

As described above, it is hard even for a skilled operator to achievethe optimum control of the tilt speed and the tilt angle with a speedytilting action. In other words, due to the great inertia force of theblade 10, an operator cannot stop the supplying of the pressurized oilto the left and right pitch cylinders 20A and 20B with a lever controlbefore the blade 10 interferes with the radiator guard or the like. Itoccurs due to a requirement of approximately 0.5 second for a responsedue to properties of oil pressure in conventional hydraulic devices.

To solve these problems, frames 15A and 15B are made longer as shown inFIG. 10B so as to achieve a longer distance between the blade 10 and thevehicle body 20x of the bulldozer 20, whereby the interference can beprevented.

Assuming that W is a weight of the bulldozer 20, Lw is a distancebetween the center of an actuating wheel 20y and a position of thecenter of gravity G, and Lf is a distance between the center of theactuating wheel 20y and a nose of the blade 10, the following relationis satisfied:

    F=(Lw/Lf)W

This relation, however, suggests a problem in that, if the distance Lfbetween the center of the actuating wheel 20y and the nose of the blade10 becomes longer, the nose force F of the blade 10 is reduced, whichlowers an operation capability. In addition, it makes the length L1longer for moving the blade 10 rotatably around point pins 15a and 15bof the frames 15A and 15B, which increases the sensitivity of themovement amount of the blade 10 for the control amount of the operatorand therefore it becomes harder to control the blade.

Accordingly, to secure the nose force F of the blade 10, the blade 10must be put into a position close to the vehicle body 20x as shown inFIG. 10A. In the Japanese Non-examined Patent Publication No. 2-204534,this applicant proposed a control device, having functions of tiltingand pitching actions of the blade 10, for this type of a bulldozer 20.

To increase the capacity of pressed earth of the blade 10 of thebulldozer 20, it is also required to increase the pitch back angle andthe tilt speed. Additionally, in greatly changing the tilt angle of theblade 10, the operator must stop the tilting action so as to prevent thetilt angle of the blade 10 from exceeding the tilt angle limit value. Atthis point, it is important to determine when the tilting action shouldbe stopped. However, since the tilt angle limit value depends upon thepitch back angle of the blade 10, it is hard for an operator todetermine it by himself. To stop the blade 10 so as not to interferewith the radiator guard, the operator must repeat the tilt action manytimes.

Furthermore, a higher tilt speed of the blade 10 causes the tilt angleto exceed the tilt angle limit value, while a lower tilt speed thereofrequires a long time for obtaining a tilt angle desired by the operator,by which it becomes harder to control the blade. This causes a need forexpensive devices such as a proportioning control valve or the like.

SUMMARY OF THE INVENTION

The present invention is provided from the viewpoint of the problems setforth above. Therefore, it is an object of the present invention toprovide a control device and a method by which interference between abulldozing blade of a bulldozer and the vehicle body can be prevented byan automatic control of precisely stopping a tilting action of the bladewhen the tilt angle reaches a predetermined tilt angle limit value evenwhen the pitch back angle of the blade is made large and the tilt speedis increased to greatly change the tilt angle.

In a control device for a blade of a bulldozer, comprising a bladeinstalled pivotably in the forward and backward directions, atrespective ends of frames on both sides of the vehicle body, the leftand right lift cylinders for controlling the movement of the bladeupwardly and downwardly, with one end removably installed in the bladeand the other end installed in the vehicle body, and the left and rightpitch cylinders for controlling the pitch of the blade with pitch dumpor pitch back actions via first and second directional control valvesand for controlling the tilt of the blade with left and right tiltactions via one of the first and second directional control valves, withone end removably installed in the blade and the other end in respectiveframes; there are provided left and right detecting means for detectingthe amount of tilt of the blade, left and right tilt limiting valveswhich are connected to operating units of the first and seconddirectional control valves, and a controller for outputting a command tothe left and right tilt limiting valves so as to stop a tilting actionof the blade when a difference between the amounts of tilt detected bythe left and right detecting means reaches a predetermined limit value.

This controller can be equipped with a delay circuit for calculating acorrection value used for initiating the stopping of the tilting actiona certain period of time before the difference between the amounts oftilt detected by the left and right detecting means reaches thepredetermined limit value.

The detecting means for detecting the amounts of tilt of the blade cancomprise yoke angle sensors for detecting the rotational angles of theyokes for supporting the left and right lift cylinders or stroke sensorsfor detecting working strokes of the left and right pitch cylinders.

In a method of the present invention for controlling the bulldozerblade, which comprises controlling the movement of the blade upwardlyand downwardly with contraction/expansion driving of the left and rightlift cylinders of the vehicle body, controlling the pitch of the bladewith pitch dump and pitch back actions by supplying pressurized oil tothe left and right pitch cylinders via the first and second directionalcontrol valves during expansion driving, and controlling the tilt of theblade with left and right tilting actions by supplying pressurized oilto either of the left and right pitch cylinders via either of the firstand second directional control valves during expansion driving; thedetecting means detect respective amounts of tilt in the leftward andrightward directions when the blade is tilted leftwardly or rightwardly,and stop the tilting action of the blade based on a command outputtedfrom the controller when a difference between the amounts of leftwardand rightward tilt reaches a predetermined limit value.

The command used for initiating the stopping of the tilting action ofthe blade is outputted from the controller a certain period of timebefore the difference between the amounts of leftward and rightward tiltreaches the predetermined limit value.

As respective amounts of tilt detected by the left and right detectingmeans, the respective rotational angles of the yokes for supporting theleft and right lift cylinders can be detected and a limit value for adifference between the yoke angles can be calculated based on an averageyoke angle of the detected yoke rotational angles. When the differencebetween the left and right yoke angles reaches the predetermined limitvalue, the left and right tilt limiting valves, connected to theoperating units of the first and second directional control valves, areautomatically turned ON so as to stop the tilting action of the blade.Otherwise, the respective working strokes of the left and right pitchcylinders can be detected and a limit value for a difference between thestrokes can be calculated based on an average stroke of the detectedworking strokes. When the difference between the left and right strokesreaches the predetermined limit value, the left and right tilt limitingvalves are automatically turned ON so as to stop the tilting action ofthe blade.

In this configuration, an average amount of tilt is obtained from theamounts of leftward and rightward tilt, detected by the left and rightdetecting means for detecting an amount of tilt of the blade; and then alimit value for a difference between the amounts of leftward andrightward tilt is calculated from the average amount of tilt based on apredetermined function. Then, it is determined whether or not thedifference between the amounts of leftward and rightward tilt is lowerthan the limit value. If the difference between the amounts of leftwardand rightward tilt is equal to or greater than the limit value, acommand signal is outputted from the controller so as to automaticallyturn ON a solenoid of one of the left and right tilt limiting valves. Ifthe difference between the amounts of leftward and rightward tilt islower than the limit value, a command signal is outputted from thecontroller so as to automatically turn OFF the solenoid of one of theleft and right tilt limiting valves.

Therefore, even when a pitch back angle of the blade of a bulldozer orthe like is made large and a tilt speed is increased to greatly change atilt angle, the tilting action of the blade is automatically controlledso as to stop when the difference between the amounts of leftward andrightward tilt reaches the predetermined tilt limit value, wherebyinterference between the blade and the vehicle body is prevented, sothat an operator can control the blade safely.

In addition, the controller can be equipped with a delay circuit forinitiating the stopping of the tilting action a certain period of timebefore the difference between the amounts of leftward and rightward tiltreaches a predetermined limit value, taking into consideration theresponse properties of hydraulic devices, whereby a tilting action ofthe blade can be stopped precisely.

At this point, the amount of tilt of the blade is detected by yoke anglesensors for detecting respective rotation angles of the yokes forsupporting the left and right lift cylinders or stroke sensors fordetecting respective working strokes of the left and right pitchcylinders.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a side view of a bulldozer according to a first embodiment ofthe present invention;

FIG. 1B is a description diagram of a yoke angle sensor shown in FIG.1A;

FIG. 2 is a blade control circuit diagram according to the firstembodiment;

FIG. 3 is a block diagram of a control device according to the firstembodiment;

FIG. 4 is a chart showing a relationship between time and a limit valuefor a difference in yoke angles according to the first embodiment;

FIG. 5 is a blade control flowchart according to the first embodiment;

FIG. 6 is a description diagram showing a excavation position, a pitchdump position, and a pitch back position of a blade;

FIGS. 7A, 7B, and 7C are description diagrams of working states of thebulldozer in the respective positions of FIG. 6;

FIG. 8 is a description diagram of an interference condition between theblade of the bulldozer and a radiator guard;

FIG. 9A is a side view of the bulldozer according to a second embodimentof the present invention;

FIG. 9B is a segmentary view of a blade control circuit according to thesecond embodiment;

FIG. 10A is a description diagram of a nose force of a conventionalbulldozer; and

FIG. 10B is a description diagram of a nose force of another type of aconventional bulldozer.

BEST MODE FOR CARRYING OUT THE INVENTION

A first embodiment of a blade control device for a bulldozer and itscontrol method according to the present invention will be describedbelow with reference to FIGS. 1A to 5.

Yokes 22A and 22B are rotatably installed on the left and right sides,respectively, of the vehicle body 20x of the bulldozer 20 as shown inFIG. 1A. One end of the left hydraulic lift cylinder 21A is fixed to theyoke 22A and the other end is pivotably connected to the left endportion of the blade 10, while one end of the right hydraulic liftcylinder 21B is pivotably connected to the right end portion of theblade 10. Yoke angle sensors 23a and 23b for detecting the rotationalangles of the yokes 22A and 22B, respectively, are installed on the leftand right sides of the vehicle body 20x. Each of the yoke angle sensors23a and 23b is installed so as to detect a yoke angle θ to a horizontalplane as shown in FIG. 1B.

FIG. 2 shows tilting, pitch dump, and pitch back operation circuits ascontrol circuits of the blade 10. However, a lifting operation circuitis omitted here since it is the same as for the conventional one. Adischarge conduit of a hydraulic pump 40A is connected to a firsthydraulic pitch cylinder 20A via a first directional control valve 35. Adischarge conduit of a hydraulic pump 41A is connected to a secondhydraulic pitch cylinder 20B via a second directional control valve 36.The hydraulic pumps 40A and 41A are of a stationary-capacity type, andan outlet circuit of an auxiliary hydraulic pump 40B is connected to anoutlet circuit of the hydraulic pump 40A via an auxiliary magnetic valve33. In addition, an outlet circuit of an auxiliary hydraulic pump 41B isconnected to an outlet circuit of the hydraulic pump 41A via anauxiliary magnetic valve 34.

A pilot pump 29 is connected to a pilot valve 28, and the pilot valve 28is connected to a control lever 25. The pilot valve 28 is also connectedto a pitch dump control valve 30 and a pitch back control valve 31. Thepitch dump control valve 30 is connected to a pitch-tilt magneticswitching valve 32 via a left tilt limiting valve 37. The pitch backcontrol valve 31 is connected to the pitch-tilt magnetic switching valve32 via a right tilt limiting valve 38. An operating unit 35a of a firstdirectional control valve 35 is connected to the left tilt limitingvalve 37, and an operating unit 35b of the first directional controlvalve 35 is connected to the right tilt limiting valve 38. Respectiveoperating units 36a and 36b of a second directional control valve 36 areconnected to a pitch-tilt switching magnetic valve 32.

The left and right yoke angle sensors 23a and 23b are connected to acontroller 50. Signals sent from the left and right yoke angle sensors23a and 23b are entered into the controller 50, and then output signalsfrom the controller 50 are entered into the left tilt limiting valve 37or the right tilt limiting valve 38 according to the signals from thesensors.

Furthermore, a pitch dump switch 27 and a pitch back switch 26 areconnected to the controller 50. The output signals sent from thecontroller 50 are entered into respective auxiliary magnetic valves 33and 34, the pitch dump control valve 30 and the pitch back control valve31, and the pitch-tilt switching magnetic valve 32. If the control lever25 is operated leftwardly, the blade 10 starts a leftward tiltingaction. If it is operated rightwardly, the blade 10 starts a rightwardtilting action.

Next, the details of the controller 50 will described referring to FIG.3. Respective signals, in the form of an output voltage .O slashed.Lfrom the left yoke angle sensor 23a and at an output voltage .Oslashed.R from the right yoke angle sensor 23b, are entered into anadder 51 in the controller 50. The adder 51 enters an average yoke angleθy signal into a first arithmetic unit 52. The first arithmetic unit 52calculates a limit value for a difference in yoke angles θ0 according tothe average yoke angle θy based on a function at a predetermined tiltangle limit value, and then outputs the signals. At this point, thefunction in the first arithmetic unit 52 is set so that, if the averageyoke angle θy in the abscissa axis is increased, the limit value for adifference in yoke angles θ0 in the ordinate axis is decreased, while ifthe average yoke angle θy is decreased, the limit value for a differencein yoke angles θ0 is increased. In other words, a tilt angle limit value(corresponding to the limit value for a difference in yoke angles θ0)depends on a pitch back angle (corresponding to the average yoke angleθy) of the blade 10.

On the other hand, respective signals, in the form of the output voltage.O slashed.L from the left yoke angle sensor 23a and the output voltage.O slashed.R from the right yoke angle sensor 23b, are entered into afirst comparator 53 in the controller 50. The first comparator 53 entersa signal of a difference in yoke angles θd directly into a secondarithmetic unit 58 and a second comparator 55, and also enters it intothe second comparator 55 via a delay circuit 54. The second comparator55 enters a signal of a correction value for a difference in yoke anglesΔd into a third comparator 56, and the first arithmetic unit 52 alsoenters a signal of a limit value for a difference in yoke angles θ0 intothe third comparator 56. The third comparator 56 outputs a differencebetween the signal of the limit value for a difference in yoke angles θ0and the signal of the correction value for a difference in yoke anglesΔd (θ0-Δd) to the second arithmetic unit 58.

The second arithmetic unit 58 calculates a difference θd-(θ0-Δd) betweenthe signal of the difference in yoke angles θd from the first comparator53 and the signal (θ0-Δd) from the third comparator 56, and if thefollowing relation is satisfied;

    θd>(θ0-Δd)

it outputs a command to turn ON a solenoid of the left or right tiltlimiting valve 37 or 38 so as to stop the tilting action of the blade10. Then, the second arithmetic unit 58 transmits the signal (θ0-Δd) atthis output of the command as θ1 to a memory 57 so that it is stored.

In addition, the second arithmetic unit 58 calculates a differenceθd-(θ1-θH) between the signal of the difference in yoke angles θd fromthe first comparator and the signal θ1 read from the memory and itscorrection value θH (corresponding to a hysteresis loss), and if thefollowing relation is satisfied;

    θd<(θ1-θH)

it outputs a command to turn OFF the solenoids of the left and righttilt limiting valves 37 and 38 so as to activate the tilting action ofthe blade 10.

At this point, the solenoids of the left and right tilt limiting valves37 and 38 are controlled so as to be turned ON to start the operation ata point B1 as shown in FIG. 4 and to be turned OFF to stop the tiltingaction of the blade 10 at a point A1 when a period of time t1 haselapsed. In other words, the blade 10 is stopped precisely within therange of the limit value for a difference in yoke angles θ0. In thisembodiment, the delay time t1 is set to approximately 0.5 second, takinginto consideration the response properties of the left and right tiltlimiting valves 37 and 38.

Next, an explanation will be made for the left and right tilting actionsof the blade 10 according to this embodiment.

If the control lever 25 is operated leftwardly as shown in FIG. 2, apilot pressure oil from the pilot pump 29 flows into the pitch dumpcontrol valve 30. At this time, both of the pitch dump switch 27 and thepitch back switch 26 of the control lever 25 remain in the OFF state,and the pitch dump control valve 30 is in its position A and thepitch-tilt switching magnetic valve 32 is also in its position A.Therefore, the pilot pressure oil passes through the position A of theleft tilt limiting valve 37 and acts on the operating unit 35a of thefirst directional control valve 35, whereby the first directionalcontrol valve 35 is switched from its neutral state to its position B.

Therefore, pressurized oil, discharged from the hydraulic pump 40A,flows into a head chamber of the first pitch cylinder 20A so as toretract the first pitch cylinder 20A. At this time, the seconddirectional control valve 36 is put in its neutral state, so that thepressurized oil from the hydraulic pump 41A is not supplied to thesecond pitch cylinder 20B, whereby the second pitch cylinder 20B is putin a stop state. Accordingly, if the first pitch cylinder 20A isretracted, the blade 10 tilts leftwardly.

During the leftward tilting action of the blade 10, signals from theleft and right yoke angle sensors 23a and 23b are entered into thecontroller 50. The controller 50 calculates the limit value for adifference in yoke angles θ0 from a predetermined function based onthese signals, and then sends a signal for turning on the left tiltlimiting valve 37 approximately 0.5 second before the difference reachesthe limit. It switches the left tilt limiting valve 37 from its positionA to its position B so as to cut off the pilot pressure oil, whereby thefirst directional control valve 35 returns from its position B to itsneutral position. Therefore, the supplying of the pressurized oil to thefirst pitch cylinder 20A is stopped, and the blade 10 stops the tiltingaction.

If the control lever 25 is operated rightwardly, the pilot pressure oilfrom the pilot pump 29 flows into the pitch back control valve 31. Atthis time, the pitch dump switch 27 and the pitch back switch 26 are putin the OFF state, and the pitch back control valve 31 is in its positionA and the pitch-tilt switching magnetic valve 32 is also in its positionA. Therefore, the pilot pressure oil passes through the position A ofthe right tilt limiting valve 38 and acts on the operating unit 35b ofthe first directional control valve 35, whereby the first directionalcontrol valve 35 is switched from its neutral state to its position A.

Therefore, pressurized oil, discharged from the hydraulic pump 40A,flows into a bottom chamber of the first pitch cylinder 20A so as toextend the first pitch cylinder 20A. At this time, the seconddirectional control valve 36 is put in its neutral state, so that thepressurized oil from the hydraulic pump 41A is not supplied to thesecond pitch cylinder 20B, whereby the second pitch cylinder 20B is putin a stop state. Accordingly, if the first pitch cylinder 20A isextended, the blade 10 tilts rightwardly.

During the rightward tilting action of the blade 10, signals from theleft and right yoke angle sensors 23a and 23b are entered into thecontroller 50. The controller 50 calculates the limit value for adifference in yoke angles θ0 from a predetermined function based onthese signals, and then sends a signal for turning on the right tiltlimiting valve 38 approximately 0.5 second before the difference reachesthe limit. It switches the right tilt limiting valve 38 to its positionB so as to cut off the pilot pressure oil, whereby the first directionalcontrol valve 35 returns from its position A to its neutral position.Therefore, the supplying of the pressurized oil to the first pitchcylinder 20A is stopped and the blade 10 stops the tilting action.

Now, an explanation will be made for the pitch back and pitch dumpactions of the blade 10 according to this embodiment.

During the pitch back or pitch dump action, a command signal from thecontroller 50 is not entered into the left and right tilt limitingvalves 37 and 38, and both of them are put in their position A (openposition).

If the pitch back switch 26 of the control lever 25 is set to ON, thepitch back control valve 31 and the pitch-tilt switching magnetic valve32 are switched from the position A to their position B, and a commandsignal from the controller 50 is entered to the auxiliary magneticvalves 33 and 34, whereby the auxiliary magnetic valves 33 and 34 arealso switched from their position A to their position B. Therefore,flows of discharge from respective auxiliary hydraulic pumps 40B and 41Bjoin discharge flows of respective hydraulic pumps 40A and 41A.

At this time, the pilot pressure oil from the pilot pump 29 is suppliedfrom the position B of the pitch back control valve 31 to the operatingunit 36b of the second directional control valve 36 via the pitch-tiltswitching magnetic valve 32 and also supplied from the position B of thepitch back control valve 31 to the operating unit 35b of the firstdirectional control valve 35. Accordingly, the first directional controlvalve 35 and the second directional control valve 36 are switched fromtheir neutral state to their position A; the pressurized oil, dischargedfrom the hydraulic pump 40A, passes through the first directionalcontrol valve 35 and flows into the bottom chamber of the first cylinder20A; and the pressurized oil, discharged from the hydraulic pump 41A,passes through the second directional control valve 36 and flows intothe bottom chamber of the second cylinder 20B. Therefore, the firstcylinder 20A and the second cylinder 20B extend simultaneously, so thatthe blade 10 quickly performs a pitch dump action.

In addition, if the pitch dump switch 27 of the control lever 25 is setto ON, the pitch dump control valve 30 and the pitch-tilt switchingmagnetic valve 32 are switched from their position A to their positionB, and a command signal from the controller 50 is entered into theauxiliary magnetic valves 33 and 34, whereby the auxiliary magneticvalves 33 and 34 are switched from their position A to their position B.Therefore, flows of discharge from the auxiliary hydraulic pumps 40B and41B join the discharge flows of the hydraulic pumps 40A and 41A.

At this time, the pilot pressure oil from the pilot pump 29 is suppliedfrom the position B of the pitch dump control valve 30 to the operatingunit 36a of the second directional control valve 36 via the pitch-tiltswitching magnetic valve 32 and also supplied from the position B of thepitch dump control valve 30 to the operating unit 35a of the firstdirectional control valve 35. Accordingly, the first directional controlvalve 35 and the second directional control valve 36 are switched fromtheir neutral state to their position B; the pressurized oil, dischargedfrom the hydraulic pump 40A, passes through the first directionalcontrol valve 35 and flows into the head chamber of the first cylinder20A; and the pressurized oil, discharged from the hydraulic pump 41A,passes through the second directional control valve 36 and flows intothe head chamber of the second cylinder 20B. Therefore, the firstcylinder 20A and the second cylinder 20B extend simultaneously, so thatthe blade 10 quickly performs a pitch back action.

Next, a blade control method of this embodiment will be described belowby using a flowchart shown in FIG. 5.

First, left and right yoke angles θ are detected by the left and rightyoke angle sensors 23a and 23b in step S1. In step S2, an average yokeangle θy between the left and right yoke angles is calculated.

In step S3, a limit value for a difference in yoke angles θ0 iscalculated from the average yoke angle θy based on a predeterminedfunction. In step S4, a correction value (or a rate of change) for adifference in yoke angles Δd is calculated. In step S5, θ1 is assumed tobe a difference (θ0-Δd) between a limit value for a difference in yokeangles θ0 and a correction value for the difference in yoke angles Δdobtained when the solenoids of the left and right tilt limiting valves37 and 38 are turned ON in step S5 and then the signal is stored in thememory 57.

In step S6, it is determined whether or not a difference in yoke anglesθd is lower than the limit value. If the difference in yoke angles θd isequal to or greater than the limit value, the control proceeds to stepS7 to determine whether or not the tilt is leftward. if the tilt isleftward, the control proceeds to step S8, so that the controller 50outputs a command signal so as to turn ON the solenoid of the left tiltlimiting valve 37. Unless the tilt is leftward, the control proceeds tostep S9, so that the controller 50 outputs a comimand signal so as toturn ON the solenoid of the right tilt limiting valve 38. Then, thecontrol returns to step S1.

If the difference in yoke angles θd is lower than the limit value instep S6, the control proceeds to step S10, so that the controller 50outputs a command signal so as to turn OFF the solenoids of the left andright tilt limiting valves 37 and 38. Then, the control returns to stepS1.

As set forth hereinabove, the average yoke angle θy is obtained from theleft and right yoke angles θ and the limit value for a difference inyoke angles θ0 is calculated based on a predetermined function; if thedifference in yoke angles θd is equal to or greater than the limitvalue, the solenoids of the left and right tilt limiting valves 37 and38 are automatically turned ON so as to stop the tilt action of theblade 10 precisely to prevent the blade 10 from interfering with thevehicle body 20x or the like.

While this embodiment has been explained for the tilting operation ofthe blade 10 in the above, it will be understood that the embodiment isalso applicable to the lift operation and the pitch back operation ofthe blade.

Next, a second embodiment of the present invention will be explainedbelow with reference to FIGS. 9A and 9B. In this embodiment, instead ofthe yoke angle sensors 23a and 23b installed on opposite sides of thevehicle body 20x in the first embodiment, stroke sensors 20a and 20b fordetecting working strokes are installed in the left and right pitchcylinders 20A and 20B for actuating a tilting action of the blade 10. Anexplanation of other configurations is omitted here except that thereference numerals are shown in the drawings since they are the same asfor the first embodiment.

The stroke sensors 20a and 20b detect the working strokes of the leftand right pitch cylinders 20A and 20B, and then the signals are enteredinto the controller 50. The controller 50 obtains an average strokevalue from the signals and then calculates a limit value for adifference in strokes. If the difference in working strokes is equal toor greater than the limit value, solenoids of the left and right tiltlimiting valves 37 and 38 are automatically turned ON so as to stop thetilting action of the blade 10 precisely to prevent the blade frominterfering with the vehicle body 20x or the like.

According to the first and second embodiments, interference of the blade10 of the bulldozer with the vehicle body 20x or the like is prevented,so that the blade does not damage them. In addition, an operator cansafely control the blade 10, and therefore he feels less fatigue, whichimproves the workability.

INDUSTRIAL APPLICABILITY

The present invention is useful as a blade control device and itscontrol method, by which interference between a blade of a bulldozer anda vehicle body can be prevented by an automatic control of preciselystopping a tilting action of the blade when a difference between amountsof tilt reaches a predetermined limit value, even when a pitch backangle of the blade is made large and a tilt speed is increased togreatly change a tilt angle.

What is claimed is:
 1. A method for controlling a blade of a bulldozer,said method comprising the steps of:controlling movement of the bladeupwardly and downwardly by driving of left and right hydraulic liftcylinders; controlling pitch of the blade with pitch dump and pitch backactions by supplying pressurized oil to left and right hydraulic pitchcylinders via first and second directional control valves during drivingof said left and right hydraulic pitch cylinders; controlling tilt ofthe blade with left and right tilting actions by supplying pressurizedoil to one of the left and right hydraulic pitch cylinders via one ofthe first and second directional control valves; detecting respectiveamounts of leftward and rightward tilt when said blade is tiltedrightward or leftward; and automatically stopping the tilting action ofsaid blade when a difference between the thus detected amounts ofleftward and rightward tilt reaches a predetermined limit value toprevent interference between the blade and the bulldozer.
 2. A method inaccordance with claim 1, wherein said step of detecting respectiveamounts of leftward and rightward tilt comprises detecting respectiverotational angles of left and right yokes supporting said left and righthydraulic lift cylinders, respectively; andwherein said step of stoppingcomprises:calculating a limit value for a difference between thusdetected rotational angles based on an average yoke angle of thedetected rotational angles; and when the difference between thusdetected left and right rotational angles reaches the thus calculatedlimit value, automatically turning ON left and right tilt limitingvalves so as to stop the tilting action of said blade.
 3. A method inaccordance with claim 1, wherein said step of detecting respectiveamounts of leftward and rightward tilt comprises detecting a workingstroke of said left hydraulic pitch cylinder and detecting a workingstroke of said right hydraulic pitch cylinder; andwherein said step ofstopping comprises:calculating a limit value for a difference betweenthus detected working strokes, based on an average working stroke of thethus detected working strokes; and when the difference between thusdetected left and right working strokes angles reaches the thuscalculated limit value, automatically turning ON left and right tiltlimiting valves so as to stop the tilting action of said blade.
 4. Amethod in accordance with claim 1, wherein the step of stopping thetilting action of said blade is initiated a certain period of timebefore the difference between the amounts of leftward and rightward tiltreaches the predetermined limit value.
 5. A method in accordance withclaim 4, wherein said step of detecting respective amounts of leftwardand rightward tilt comprises detecting respective rotational angles ofleft and right yokes supporting said left and right hydraulic liftcylinders, respectively; andwherein said step of stoppingcomprises:calculating a limit value for a difference between thusdetected rotational angles based on an average yoke angle of thedetected rotational angles; and when the difference between thusdetected left and right rotational angles reaches the thus calculatedlimit value, automatically turning ON left and right tilt limitingvalves so as to stop the tilting action of said blade.
 6. A method inaccordance with claim 4, wherein said step of detecting respectiveamounts of leftward and rightward tilt comprises detecting a workingstroke of said left hydraulic pitch cylinder and detecting a workingstroke of said right hydraulic pitch cylinder; andwherein said step ofstopping comprises:calculating a limit value for a difference betweenthus detected working strokes, based on an average working stroke of thethus detected working strokes; and when the difference between thusdetected left and right working strokes angles reaches the thuscalculated limit value, automatically turning ON left and right tiltlimiting valves so as to stop the tilting action of said blade.
 7. Abulldozer comprising:a vehicle body; a left frame having a front end anda rear end with the rear end of said left frame being pivotally mountedto a left side of said vehicle body; a right frame having a front endand a rear end with the rear end of said right frame being pivotallymounted to a right side of said vehicle body; a blade pivotally mountedto each of the front ends of said left frame and said right frame so asto permit pivoting of said blade in a forward direction and in abackward direction; a left hydraulic lift cylinder and a right hydrauliclift cylinder for controlling movement of the blade upwardly anddownwardly, each of said left hydraulic lift cylinder and said righthydraulic lift cylinder having a first end and a second end, the firstend of each of said left hydraulic lift cylinder and said righthydraulic lift cylinder being removably and pivotally installed to theblade, the second end of each of said left hydraulic lift cylinder andsaid right hydraulic lift cylinder being pivotally mounted to saidvehicle body; a first directional control valve having operating unitsfor operating said first directional control valve; a second directionalcontrol valve having operating units for operating said seconddirectional control valve; a left hydraulic pitch cylinder and a righthydraulic pitch cylinder for controlling pitch of the blade with pitchdump or pitch back actions via said first directional control valve andsaid second directional control valve and for controlling the blade withleft and right tilt actions via one of said first and second directionalcontrol valves, each of said left hydraulic pitch cylinder and saidright hydraulic pitch cylinder having a first end and a second end, thefirst end of each of said left hydraulic pitch cylinder and said righthydraulic pitch cylinder being removably and pivotally installed to theblade, the second end of each of said left hydraulic pitch cylinder andsaid right hydraulic pitch cylinder being removably and pivotallyinstalled to a respective one of said left frame and said right frame; aleft detector for detecting an amount of tilt of said blade; a rightdetector for detecting an amount of tilt of said blade; a left tiltlimiting valve and a right tilt limiting valve connected to operationsunits of said first and second directional control valves; and acontroller for outputting a command to the left and right tilt limitingvalves so as to stop a tilting action of said blade when a differencebetween amounts of tilt detected by said left detector and said rightdetector reaches a predetermined limit value selected to preventinterference between the blade and the vehicle body.
 8. A bulldozer inaccordance with claim 7, wherein the second end of said left hydrauliclift cylinder is pivotably mounted to said vehicle body via a leftyoke;wherein the second end of said right hydraulic lift cylinder ispivotably mounted to said vehicle body via a right yoke; wherein saidleft detector is a yoke angle sensor for detecting a rotational angle ofsaid left yoke; and wherein said right detector is a yoke angle sensorfor detecting a rotational angle of said right yoke.
 9. A bulldozer inaccordance with claim 7,wherein said left detector is a stroke sensorfor detecting a working stroke of said left hydraulic pitch cylinder;and wherein said right detector is a stroke sensor for detecting aworking stroke of said right hydraulic pitch cylinder.
 10. A bulldozerin accordance with claim 7, wherein said controller is equipped with adelay circuit for calculating a correction value used for initiating astopping of the tilting action of the blade a certain period of timebefore the difference between amounts of tilt detected by said left andright detectors reaches the predetermined limit value.
 11. A bulldozerin accordance with claim 10, wherein the second end of said lefthydraulic lift cylinder is pivotably mounted to said vehicle body via aleft yoke;wherein the second end of said right hydraulic lift cylinderis pivotably mounted to said vehicle body via a right yoke; wherein saidleft detector is a yoke angle sensor for detecting a rotational angle ofsaid left yoke; and wherein said right detector is a yoke angle sensorfor detecting a rotational angle of said right yoke.
 12. A bulldozer inaccordance with claim 10, wherein said left detector is a stroke sensorfor detecting a working stroke of said left hydraulic pitch cylinder;andwherein said right detector is a stroke sensor for detecting aworking stroke of said right hydraulic pitch cylinder.